My browser / operating system: Windows NT 10.0, Chrome 91.0.4472.114, No Flash versions detected
Basically the text doesn't show in the costume editor but it on stage it is.
Steps to replicate:

1. Create some text
   
   
2. Reselect it (click off then select it again
   
3. Delete all the text
   
   
4. Unselect it
   

I have a feeling this is intentional because it's been in here for a long time but I can't be sure :\

Can replicate. Firefox 89 on macOS Big Sur 11.4


Another one! Right after this bug happens, click on the stage, then after a second, click back to that sprite, and the text should be back!